ON ARSENIC 115 



the nitre begins to grow thick, and will no longer flow in 

 the same degree of heat. If a part of the mass thus 

 obtained be dissolved in water, it will redden paper stained 

 with lacmus. After evaporating the solution, a quantity of 

 arsenical neutral salt is obtained with a little nitre. But 

 if the fire is a little increased, the mass begins to boil and 

 yield red vapours. When it grows thick, and the boiling 

 with this degree of fire is over, a solution of it in water 

 will not change paper coloured by lacmus, but syrup of 

 violets will be turned green ; yet no alkaline taste will 

 be perceived. This solution does not shoot into crystals 

 (Sec. vi. (a)). 



If at last the fire be increased to such a degree that 

 the crucible and the saline mass are perfectly red hot, a 

 new ebullition begins, and a srnell of aquafortis is perceived ; 

 at last the whole melts into a transparent mass. If this 

 be dissolved in water, it proves to be strongly alkaline ; but 

 it contains as much acid of arsenic as the first solution. If 

 one part of arsenic be exposed with two parts of nitre in 

 a glass retort to such a degree of heat as melts the retort, 

 no neutral arsenical salt is obtained, but an alkaline mass 

 mixed with some acid of arsenic. From this experiment it 

 appears that Mr. Macquer applied too strong a heat, and 

 obtained, therefore, no neutral arsenical salt in the crucible. 

 But it will be asked, Why a greater degree of heat alkalises 

 the mass, whereas nitre in an equal degree of heat, or neutral 

 arsenical salt in a still greater, is not alkalised (Sec. vi. (c)) ? 

 The following experiment gives the solution of this problem. 

 If acid of arsenic be saturated with alkali of tartar till 

 the solution of lacmus is no longer reddened, and the 

 solution be afterwards evaporated to dryness, and then dis- 

 tilled with double the quantity of nitre in a glass retort 

 furnished with a receiver,' the nitrous acid will be expelled 



